The next time you
feel annoyed because it is raining, again, just think what life would be like if there were no rain. No
grass, no flowers, no trees, no birds, no bees, no life.

Hedgerow after rain

Many places in the
world cannot support any life because they have no rainfall.

What is
rain?

Evaporation and
condensation

·The atmosphere on the
planet Earth (the air around us) is full of water vapour, which you cannot see.

·Vapour is what water
turns to when it is heated, like steam out of a boiling kettle.
This process is called evaporation.

·Most of the water
vapour in the atmosphere has evaporated from the world’s oceans. Of
course the oceans do not boil like a kettle but the heat of the planet earth means that there is constant
evaporation of moisture from its surface.

·When the air is
warm, it can hold a lot of water vapour. However, as the warm air
rises it meets colder air. Cold air cannot hold as much water
vapour as warm air.

·As the warm air
meets the cold air, the water vapour condenses, which means that it
turns into tiny drops of water or ice.

·These particles
(tiny drops of water or ice) join together to form clouds.

·As the clouds grow
in size the water drops join together and become heavy. They then
fall to the earth and it rains.

What makes
it rain?

Before clouds can
form, the air has to rise.

·As we have seen, warm
air naturally rises. This is because as the air heats, it expands and
rises upwards. This is known as convection.

·Air is carried upwards
when winds meet obstacles such as mountains. This is why we often see clouds around mountain tops.

·A mass of cold air can
act just like a mountain if it meets warmer air. We call this mass of
cold air a cold front. The warmer air is carried above the cold air. As
the warm air cools, the water vapour in it condenses and turns into rain.

The
wettest parts of the Earth

The wettest areas
are those reached by winds which have blown across the oceans for very long distances. The rain is heaviest in warm, tropical climates where the heat increases
the rate of evaporation.

For example, in
parts of Burma more than 390 inches of rain fall in a year (the average rainfall in London is 32 inches and
in New York 49 inches).

The monsoon
describes a great rush of moist air from the ocean to the land.
Many tropical countries have monsoons, but the best known monsoon is the Indian summer monsoon. In June the Trade Winds reach India. Since they have crossed the Indian Ocean, they are full of water
vapour. When they meet the hot dry air over India they cause
tropical storms, which lead to heavy rainfall over much of the country.

The rainfall can
cause rivers to overflow and there is widespread flooding.

How big is
a raindrop?

Raindrops are
really very small, usually about one hundredth of an inch. Even
the largest raindrops are only one tenth of an inch in diameter (from side to side).

Rain falls from
the clouds at a speed of between 2 miles per hour, for very fine drops, and 18 miles per hour for the largest drops.